Fuel cells are devices that generate electricity by electrochemical reactions between fuels such as hydrogen and air (oxygen) and have high power generation efficiency since they can directly convert chemical energy into electricity. In particular, solid oxide fuel cells (hereinafter referred to as SOFCs), which have operating temperatures of 700° C. or higher, particularly about 800° C. to 1,000° C., have high reaction rates and are easy to handle since all cell elements are solid. The application of SOFCs, however, is limited to large power generation systems and household power generation because of their very high operating temperatures. With today's growing need for energy conservation, there is a need to extend the range of applications of SOFCs, which have high power generation efficiency, low noise, low emissions of environmentally harmful substances, and simple cell constructions.
SOFCs have high operating temperatures so that oxide ions can migrate through ceramic solid electrolytes. Accordingly, research has been directed to protonic ceramic fuel cells (PCFCs), which use hydrogen ions (protons), rather than oxide ions, as charge carriers since protons can migrate in the medium-temperature range (e.g., 400° C. to 600° C.). These fuel cells require protonically conductive solid electrolytes. Examples of known protonically conductive solid electrolytes include compounds having a perovskite structure, for example, metal oxides such as yttrium-doped barium zirconate (hereinafter referred to as BZY) and yttrium-doped barium cerate (hereinafter referred to as BCY) (PTL 1).
Metal oxides are susceptible to moisture. In particular, metal oxides containing cerium have been reported to be more susceptible to moisture than metal oxides containing zirconium (NPL 1).